Ungrounded power distribution systems are widely used, especially at medium voltage levels, e.g., less than 50 kV. The ungrounded power distribution system is a three-phase three-wire system. The windings of three-phase transformers within the system use either ungrounded WYE or DELTA connection, and three-phase loads are DELTA connected.
For a number of applications, it is necessary to determine the power consumption of loads at an instant of time, because load profiles may not accurately represent the current power consumption. For example, one of the frequently occurred faults in the power distribution system includes a single-phase-to-ground short circuit fault. For a grounded distribution system, the fault currents are usually greater than the load currents, so the impacts of loads can be ignored when determining the location of the fault. However, for an ungrounded distribution systems, the fault currents are usually less than the load currents, therefore the accuracy of the determination of the location of the fault depends on the accuracy of determination of the power consumption of the loads.
Several methods have been used for estimating loads in ungrounded distribution systems. For example, a method of load forecasting for a present day described in US 20120221158 A1 includes obtaining past observed load values of at least three earlier days and identifying a relationship between the present day's load forecast and the past observed load values including unknown weights associated with the past observed load values. Another method described in U.S. Pat. No. 7,979,222 employed load profiles in combination with short-term load estimations to forecast expected loads on a distribution network. The predicted loads may then be used by power restoration controllers when performing back-feed capacity checks.
All those methods have limitations either in processing time or estimation accuracy when applied to real time applications in the ungrounded power distribution systems.